Identifying if merge queue branch is up-to-date with base branch

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tl;dr: If we are merging e.g. feature/foo into main in a merge queue, is it possible to identify the feature/foo branch (rather than `gh-readonly...) from within the merge queue job?

Context:

We are using a merge queue, and would like to skip checks in the merge queue if the merging branch is up-to-date when it enters the queue (to avoid duplicating the checks which must have just happened to get it into the merge queue).

Since https://github.com/orgs/community/discussions/48792 (and the threads linked there) suggest there is no option to do this automatically, we have been investigating doing this manually.

We added the following step to our merge queue workflow:

if ${{ github.event_name == 'merge_group' }}; then
  base_ref=${{ github.event.merge_group.base_ref }}
  base_ref="${base_ref#refs/heads/}"
  head_ref=${{ github.event.merge_group.head_ref }}
  head_ref="${head_ref#refs/heads/}"
  git fetch --depth 2 origin $base_ref
  git fetch --depth 2 origin $head_ref
  up_to_date=true
  git merge-base --is-ancestor origin/$base_ref origin/$head_ref || up_to_date=false
  echo "skip_checks=$up_to_date" >> ${GITHUB_OUTPUT}
fi

this is intended to identify if merge_group.head_ref is up-to-date with merge_queue.base_ref, and to set a skip_checks flag that we can then use to skip checks if the branch is up-to-date.

The problem is that head_ref is a gh-readonly... branch, and this branch is always up-to-date with base_ref, so skip_checks is always set to true.

If we could identify the original branch which is being merged (e.g. feature/foo rather than gh-readonly...), I think this skip check would work, but it doesn't seem possible to get this information from within the merge queue. All of the members of event.merge_group seem to relate to gh-readonly... rather than feature/foo.